Glossary term
Hydrostatic Pressure
Pressure in a stationary fluid caused by fluid weight, depth, density, gravity, and any applied surface pressure.
Definition
quantityHydrostatic pressure is the pressure at a point in a stationary fluid due to the weight of the fluid column above it.
For an incompressible fluid of constant density, hydrostatic pressure increases linearly with depth according to p = p0 + rho g h. It acts normal to surfaces and is independent of direction at a point. Hydrostatic pressure is used in tank design, dams, submerged structures, manometry, diving, ballast systems, pressure testing, groundwater engineering, and many fluid-storage problems.
Hydrostatic pressure is the pressure in a fluid that is not moving relative to the container or reference frame. For a constant-density fluid, the pressure at depth h below a free surface is:
where p_0 is pressure at the free surface, \rho is fluid density, g is gravitational acceleration, and h is vertical depth. The pressure increase depends on depth, not on the shape of the container. A narrow column and a wide tank with the same fluid height produce the same pressure at the same depth.
Surface forces
Hydrostatic pressure acts normal to a surface. On a horizontal surface at uniform depth, the force is simply pressure times area. On a vertical wall, pressure increases with depth, so the resultant force acts below the centroid of the area. This matters for dams, tank walls, gates, ship hulls, retaining structures, underwater windows, and submerged equipment.
The centre of pressure is lower than the centroid for a vertical rectangular surface because the pressure distribution is triangular. Designers must calculate both the magnitude and location of the resultant force to size supports, hinges, seals, and wall thickness.
Gauge and absolute pressure
Hydrostatic calculations can use gauge or absolute pressure, but the reference must be consistent. For a vented tank, gauge pressure at the free surface is zero, so gauge pressure at depth is \rho g h. Absolute pressure at the same point includes atmospheric pressure. For vacuum tanks, pressurized vessels, or closed systems, the surface pressure term cannot be ignored.
Pressure head expresses pressure as an equivalent fluid height:
This is useful in hydraulics, manometers, pumps, and water systems because it links pressure directly to elevation.
Limitations
The simple linear formula assumes a static fluid with constant density. For gases, deep compressible fluids, stratified fluids, accelerating containers, rotating fluids, waves, sloshing, and flowing systems, pressure distribution can differ significantly. In moving systems, dynamic pressure, head losses, and transient effects such as water hammer may dominate over simple hydrostatics.
Common mistakes include using total depth instead of vertical depth, forgetting atmospheric or vessel pressure, applying water density to a different fluid, and using hydrostatic pressure where the fluid is actually accelerating or flowing.